In many situations and for many reasons it may be desirable to locate a radio transmitter using passive techniques (i.e. analyzing signals received from the unknown location) rather than active techniques (i.e. initiating signals to detect and identify the unknown location). For instance, passive techniques may be preferable if many different transmitters are located in the same general area, and a particular one of these many transmitters needs to be located. Further, passive techniques may be preferable if a transmitter, at an unknown location, is particularly difficult to detect and uniquely identify in its environment.
Locating a transmitter that is mobile and which has subsequently moved from its last known location can be particularly troublesome. Mobility, however, is an increasingly common feature of communications systems. For the case in point, consider a mobile cellular phone. With specific regard to the user of a cellular phone in a cellular telecommunications network, the ability to locate the particular cellular phone can be advantageous for several reasons. To name but a few of these advantages, information as to the exact location of a subscriber unit would be helpful for: 1) emergency 911 operators, 2) fleet tracking, 3) geoselective billing, 4) wide-area traffic monitoring, and 5) motorist assistance. In one aspect, the ability to locate a transmitting station relies on rather simple geometry.
Geometrically, several techniques can be used to find and locate an unknown point on the earth's surface. Perhaps, the simplest technique requires only range and bearing information. Thus, given that there is some known point from which measurements can be taken, it is possible to locate an unknown point if both the distance (range) and direction (bearing) from the known point to the unknown point can be determined. For another technique, if range can not be determined, bearings from two known points can be used to locate the unknown point. According to still another technique, if bearings can not be determined, the range differences from three known points, or measurements proportional to these range differences, can be used to locate the unknown point. Not surprisingly, various techniques for using electronic equipment to locate unknown points have been proposed.
One well known and widely used technique for locating the position of a mobile terminal is to have the terminal broadcast information about its location. For example, this can be done using the GPS system. This technique, although very effective and very accurate, increases the cost of the mobile terminal significantly. A second technique involves what is well known as Time Difference of Arrival (TDOA). For TDOA, the arrival time of a transmitted signal is determined at several known points. These arrival times are then used to locate the unknown point. TDOA, however, is vulnerable to errors induced by cochannel interference and multi-path propagation. Yet another technique involves determining the Angle of Arrival (AOA) of an emitter signal at several known points. Using AOA, however, additional specifically designed and calibrated antennas, with attendant increased costs, are required. In contrast with these previously used techniques, the present invention recognizes less costly systems, using uncalibrated antennas, can be used with equally effective results.
As implied above, by comparing range differences proportional measurements from a transmitter to several known points, the unknown location of the transmitter can be determined. To do this, however, it is necessary, to first determine the direct path component of the transmitted signal. This is so because it is the direct path component which has traveled the direct straight line distance from the transmitter to the receiving antenna. Consequently, cochannel interference and multi-path signals or reflections need to be eliminated or ignored. Typically, this is done by establishing a beamformer which serves to isolate the direct path component of the signal of interest.
Heretofore, several types of beamformers have been proposed. Most pertinent to the present invention are the so-called "blind" beamformers. These "blind" beamformers include systems which typically use 1) Eigenvector Techniques, 2) a Constant Modulus Algorithm, or 3) Least Squares Techniques. Such systems are currently available, and are variously applied by skilled artisans where appropriate. All of these "blind" beamformers, however, share one weakness when used to estimate propagation time. Namely, this weakness is that strong, highly correlated multi-path components will introduce a positive bias that the system is not able to correct.
In light of the above, it is an object of the present invention to provide a wireless system for determining the location of a signal emitter which is not influenced by the positive bias of strong, highly correlated multipath components. Another object of the present invention is to provide a wireless system for determining the location of a signal emitter which is able to retrieve range difference proportional measurements from the direct path component of a transmitted signal using uncalibrated antenna arrays. Still another object of the present invention is to provide a method for identifying the direct path component of an emitter signal using the phase delay characteristics of a cyclostationary feature in the signal, referred to herein as a Cyclic Phase Minimizer. Yet another object of the present invention is to provide a wireless system and method for using such a system that is relatively easy to employ, simple to use, and comparatively cost effective.